Measuring instruments with electromagnetic load compensation include, for example, balances operating on the pulse compensation principle, that is balances in which the compensation coil receives pulses for a time interval which depends on the load on the balance, or balances which operate in accordance with a charge balancing principle.
It is particularly important in this type of measuring instrument to detect malfunctions which falsify the measured result, without actually interrupting the operation of the instrument. It is thus necessary to monitor the constant current source (operating constant current source) or the corresponding reference voltage source and reference resistance. In conventional measuring instruments, a second constant current source (test constant current source) or a second reference voltage source were supplied. At a given time, the operator would then switch from the operating constant current source to the test constant current source and check whether the results using the two sources were within a given tolerance.
For this purpose the operating and test voltages or currents had to be exactly equal. This requires a considerable outlay which must, in addition, be repeated several times during the life of the instrument if the long term stability of the two sources is not the same. Also, since switching from the operating to the test voltage or current source had to be initiated by the operator, changes in the former could pass unnoticed over long time periods.